Linear aperture deposition apparatus and coating process

ABSTRACT

A linear aperture deposition apparatus and process are provided for coating substrates with sublimed or evaporated coating materials. The apparatus and process are particularly suited for producing flexible films having an optical interference coating with a very high surface thickness uniformity and which is substantially free of defects from particulate ejection of a source material. The apparatus includes a source box containing a source material, a heating element to sublime or evaporate the source material, and a chimney to direct the source material vapor from the source box to a substrate. A flow restricting baffle having a plurality of holes is positioned between the source material and the substrate to confine and direct the vapor flow, and an optional floating baffle is positioned on the surface of the source material to further restrict the vapor flow, thereby substantially eliminating source material spatter.

[0001] This application claims the benefit of priority to U.S.Provisional Application Ser. No. 60/108,187, filed on Nov. 12, 1998, thedisclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the field of vacuumdeposition processes, and more particularly to a linear aperturedeposition apparatus and coating process for coating wide substratematerials.

[0004] 2. Relevant Technology

[0005] Optical interference coatings are useful for controlling thereflection, transmission and/or absorption of a selected wavelengthrange of light. These coatings consist of a plurality of alternatinglayers having a predetermined thickness less than the selectedwavelength range. Additionally, the layers have a significant differencein refractive index and are controlled to a predetermined thickness.Suitable materials for optical interference coatings are primarilydielectric materials which have a refractive index range of about 1.4 toabout 2.4, which is wavelength dependent, and a very small opticalabsorption coefficient. In some applications, thin layers of metalfilms, which have large absorption coefficients, are combined with thedielectric material layers.

[0006] The economical production of these coatings is frequently limitedby the thickness uniformity necessary for the product, the number oflayers, and the deposition rate of the coating materials. The mostdemanding applications generally require that the deposition occur in avacuum chamber for precise control of the coating thickness and theoptimum optical properties. The high capital cost of vacuum coatingequipment necessitates a high throughput of coated area for large-scalecommercial applications. The coated area per unit time is proportionalto the coated substrate width and the vacuum deposition rate of thecoating material.

[0007] A process that can utilize a large vacuum chamber has tremendouseconomic advantages. Vacuum coating chambers, substrate treating andhandling equipment, and pumping capacity, increase in cost less thanlinearly with chamber size; therefore, the most economical process for afixed deposition rate and coating design will utilize the largestsubstrate available. A larger substrate can generally be fabricated intodiscrete parts after the coating process is complete. In the case ofproducts manufactured from a continuous web, the web is slit or sheetcut to either a final product dimension or a narrower web suitable forthe subsequent manufacturing operations.

[0008] The manufacturing cost of the product is ultimately limited bythe specific performance requirements that limit the maximum depositionrate. For example, if the required uniformity of coating on a continuousweb or film is 1% or less over 12 inches of width, one would generallyoperate the source at the highest deposition rate, R_(max), that couldconsistently yield the requisite 1% uniformity over the 12-inch width.If operation at that deposition rate degraded another specifiedcharacteristic, such as the maximum defect size, below a minimumacceptable value, then the deposition rate would be lowered to R₁, whereR₁<R_(max).

[0009] Continuing with this example, further cost reduction could beachieved if the coating were deposited on substrates having widths thatare multiples of 12 inches; i.e., 24 inches, 36 inches, etc. Forexample, if a 36-inch-wide source achieved 1% uniformity at depositionrate R₁, it would cost less to coat a 36-inch-wide substrate and slit itto a final width of 12 inches than to coat a 12-inch-wide substrate,because three times as much material would be produced by the widercoating machine. A wider coating machine would cost less than threetimes the cost of a 12-inch coating machine, perhaps only 50% more.However, this advantage would only be realized if the 36-inch sourcecould deposit the coating with 1% uniformity over the entire 36-inchsubstrate width at a rate, R₂, which is greater than or equal to R₁,without exceeding the maximum defect size.

[0010] Therefore, in the case of continuous coating equipment, in whicha substrate of a fixed width is transported over each source to depositthe coating design, simultaneously improving the uniformity of thesource and the deposition rate without degrading the film properties,will have a profound economic benefit.

[0011] Two techniques are commonly used in the physical vapor depositionof coating materials. These are sputtering and thermal evaporation.Thermal evaporation readily takes place when a source material is heatedin an open crucible within a vacuum chamber when a temperature isreached such that there is a sufficient vapor flux from the source forcondensation on a cooler substrate. The source material can be heatedindirectly by heating the crucible, or directly by a high currentelectron beam directed into the source material confined by thecrucible.

[0012] Magnetron sputtering adapts well to coating wide substrates withmetal layers. The length of the magnetron assembly is selected such thatthe sputtering racetrack exceeds the substrate width by several inchesat each edge, wherein this central portion of the racetrack provides auniformity in thickness that is typically less than about 5%. However,magnetron sputtering equipment is relatively expensive, is limited tomaterials that can be readily formed into solid targets, and hasdeposition rates that are generally inferior to those of thermalevaporation technologies, especially for metal compounds that are usefulas optical coating materials.

[0013] A Knudsen cell evaporation source is an isothermal enclosure orcrucible with a small orifice that confines the source material andrequires the vapor to diffuse out of the orifice. The inside of the cellis large compared with the size of the orifice to maintain anequilibrium interior pressure.

[0014] The enclosed nature of the Knudsen cell reduces the likelihoodthat particulate ejected by the solid source material, commonly known asspatter, will reach the substrate either to cause damage or to beembedded therewithin. It is generally believed that such spatter isgenerated by the non-uniform heating of a granular or otherwisenon-homogeneous source material whereby locally high pressures cause theejection of the most friable portions of the source material. Spatter issevere in source materials with a low thermal conductivity and havingretained moisture, air or other high vapor pressure components, andincreases with the heating rate due to increased temperaturedifferentials.

[0015] Thermal evaporation generally has been adapted to coating widesubstrates by two methods. The most common method is to create a lineararray of point sources, each point source being a small crucible havingeither a common or individual heating source. An alternative techniqueis to confine the source material in an elongated crucible and sweep anelectron beam over the entire length of the crucible in order touniformly heat the source material. A linear crucible must uniformlyheat the coating material to achieve a uniform flux of coating materialvapor across the entire substrate width.

[0016] The principle of a Knudsen cell has also been applied to coatingwide areas. The cell enclosure is a tube or rectangle matching the widthof the substrate and having a constricted slit along its entire length.Although a tubular Knudsen cell is easy to fabricate, it can bedifficult to uniformly fill with solid source material, especially whenthe slit is relatively narrow with respect to the width of the sourcematerial particles. U.S. Pat. No. 5,167,984 to Melnyk et al., disclosesfurther details optimizing a tubular Knudsen cell. The crucible has anopen end suitable for alignment of a hollow cylindrical insertcontaining the source material. The source was designed and optimizedfor depositing chalcogenide photoconductive compounds and organicphotoconductive materials.

[0017] U.S. Pat. No. 4,094,269 to Malinovski et al., discloses atank-shaped source with a rectangular slot on its surface for the vapordeposition of silver halide compounds onto glass substrates andpolyester substrates.

[0018] Prior art methods of depositing dielectric materials from eithera series of electron beam point sources or linear crucibles havenumerous limitations, especially for the economical production ofoptical interference coatings. They typically utilize less than about15% of the source material evaporated, the balance of the sourcematerial being deposited on the coating chamber interior and maskingfixtures. Both the chamber and masking fixtures must be cleanedperiodically, resulting in lower utilization of the capital equipmentcapacity and higher material costs.

[0019] Masking fixtures are commonly used to correct for sourcenon-uniformity in the direction transverse to the substrate's linearmotion, a direction referred to herein as the “cross web direction”.(The use of the term “cross web direction” is not meant to limit thepresent invention to plastic films or web products as the coatedsubstrate.) The mask decreases the deposition rate further, to theminimum value along the source width.

[0020] Attempts to increase deposition rate by increasing source powerinput, such as electron beam current, result in either an unstable meltpool, or can further decrease the coating uniformity or increase therate of particulate ejection, i.e., spatter, from solid or subliming andliquid materials. Either coating uniformity or surface qualityconsiderations always limit the deposition rate.

[0021] The development of a linear source for the evaporation of higherrefractive index materials has been a particularly elusive problem.While some successes have been obtained in depositing silicon monoxideand materials that sublime at a temperature less than about 900° C.,this limits the available refractive index to a range from about 1.6 toabout 1.9.

[0022] Many of the more useful high index materials in optical coatings,such as titanium dioxide, zirconium dioxide and niobium pentoxiderequire heating to a much higher temperature to obtain the necessaryvapor pressure for vacuum coating, typically from about 1800° C. togreater than about 3500° C.

[0023] There have been specific attempts to adapt forms of linearcrucible sources to coating flexible plastic film in a continuous rollform. That is, the substrate is continuously unwound in the vacuumchamber to transport it over the evaporation source(s), the substratebeing disposed around a large cooling drum, where it is brought into thedesired spatial proximity to the linear crucible.

[0024] In U.S. Pat. No. 5,239,611 to Weinert, a crucible device isdisclosed wherein a plurality of radiant heaters is disposed above thematerial to be evaporated. A series of outlets between the radiantheaters are in vapor communication with material being evaporated.

[0025] European Patent Application Nos. EP 0652303 and EP 0652302 toBaxter et al., disclose linear crucible evaporation sources. Referringto FIG. 1A, a prior art apparatus 20 is shown which corresponds to theevaporation source disclosed in the Baxter applications. The apparatus20 has an evaporator 22 and a chilled drum 24 which transports a websubstrate 26 to be coated across a deposition zone 28. The evaporator 22includes a crucible 30, which is heated from below by a heating element32. The crucible 30 is contained in a retort 34 having a lid 36, whereinlid 36 has a plurality of outlet nozzles 38 disposed in arcuateconformance to chilled drum 24. Referring to FIGS. 1B and 1C, outletnozzles 38 may be a plurality of holes or narrow slots oriented in thesubstrate transport direction, i.e., perpendicular to the long axis ofthe source.

[0026] A linear evaporation source for use in web coating equipment isavailable commercially from General Vacuum Equipment Corp. ofBirmingham, England. A cross-sectional diagram of this source isprovided in FIGS. 2A and 2B. Referring to FIG. 2A, a coating apparatus40 includes a drum 42 and a source 44. The source 44 includes a crucible46 containing a source material 48. Vaporized source material travelsfrom crucible 46 to a deposition zone 50 via a chimney 52. A fixedmonolithic insert 54 is placed between source material 48 and chimney 52at the top of crucible 46. An enlarged view of crucible 46, insert 54and chimney 52 is shown in FIG. 2B.

[0027] Furthermore, prior art methods of coating plastic films arefrequently limited to specific substrates depending on the heating loadof the source and the substrate's heat deformation temperature. Thislimits the choice of coating materials that can be evaporated and themaximum coating thickness. The coating thickness (per pass by coatingsource) is limited in that a minimum web speed must be exceeded to avoidoverheating the substrate.

[0028] Continuous vacuum coating of plastic substrates requires numerouscompromises to be made in product cost, composition, performance orquality due to deposition source technology. There has been anespecially acute need for an efficient thermal evaporation source forcoating plastic films with high refractive index optical material, i.e.,a refractive index greater than about 1.7, and preferably greater thanabout 1.9.

[0029] Zinc sulfide (ZnS) is a useful high refractive index opticalmaterial in both visible and infrared wavelengths. Its relatively lowsublimation temperature range, from about 1000° C. to about 1900° C.,would suggest that it is an ideal material for plastic web coating, butit has two inherent material problems. The deposition temperature mustbe well-controlled to minimize the decomposition of ZnS to zinc andsulfur atoms in the vapor state. Dissociation results in asub-stoichiometric film, having an excess of zinc, when the zinc andsulfur recombine to form a solid film. Sub-stoichiometric ZnS hasundesirable optical absorption. Also the uncontrolled dissociationresults in residual sulfur compounds on vacuum chamber components, mostnotably in the vacuum oil, and an undesirable odor. Further, chemicalreactions of the excess sulfur may accelerate the deterioration ofvarious vacuum components.

[0030] Thus, there is a need for efficient linear evaporation sourcesthat do not suffer from the foregoing disadvantages.

SUMMARY AND OBJECTS OF THE INVENTION

[0031] It is an object of the present invention to provide an apparatusand process for uniform vacuum coating of wide substrates from sourcematerials, primarily but not limited to metals and metallic compounds.

[0032] Another object of the present invention is to provide asublimation and evaporative coating apparatus and process that satisfiesthe need for high and stable deposition rates, thickness control, highcoating quality, and efficient use of the source materials.

[0033] Another object of the invention is to obtain a wide variety offunctional and multilayer coatings without damage totemperature-sensitive substrates by radiation from the coating sourcematerials and hot components.

[0034] A further object of the invention is to provide a coating sourceapparatus that is compact, being adaptable to a variety of substratetypes and/or coating equipment configurations by adapting a cooperativearrangement of serial and/or parallel arrays of multiple sources.

[0035] Another object of the invention is to utilize a source materialefficiently and obviate the need for uniformity control masking bydepositing the coating on the substrate rather than the vacuum chamber.

[0036] Yet another object of the invention is to provide coatings havinga high optical quality and being essentially free of defects fromparticulate ejected by the source material.

[0037] Still another object of the invention is to provide coatingshaving optical constants desirable for application in multilayer opticalinterference products, especially solar control films and lightinterference pigments, wherein a near bulk property refractive index isobtained without significant optical absorption.

[0038] Still another object of the invention is to provide a source thathas a fast temporal response to changes in input power, permittingcontinuous control of the deposition rate and providing the economicadvantages of a short time for heating up and cooling down.

[0039] In accordance with these and other objects, the present inventionprovides a linear aperture deposition apparatus and process for coatingsubstrates with sublimed or evaporated coating materials. The apparatusincludes a source box containing a source material, a heating element tosublime or evaporate the source material, and a chimney to direct thesource material vapor from the source box to a substrate. The chimneyhas a rectangular vapor outlet slot for directing the source vapor fromthe source box to a wide substrate. A flow restricting baffle having aplurality of holes is positioned between the source material and thesubstrate to confine and direct the vapor flow, and an optional floatingbaffle is positioned on the surface of the source material to furtherrestrict the vapor flow, thereby substantially eliminating sourcematerial spatter. The floating baffle is adapted to maintain itsposition on the upper surface of the source material, as the sourcematerial evaporates. The floating baffle has openings that are arrangedin a co-operative association with the flow restricting baffle holes toblock particulate ejected from the source material. The foregoingelements are enclosed within a containment vessel adapted for conductivecooling, whereby excess heating of the substrate and other parts of thevacuum chamber are substantially prevented.

[0040] A process of the invention provides for physical vapor depositionof a source material onto a substrate utilizing the above describedapparatus. A source material within a source box is heated such that thesource material forms a vapor. The flow of vapor out of the source boxis restricted in order to form a vapor plume substantially free of solidparticulate source material. A substrate to be coated is transportedacross the vapor plume to cause the source material to coat thesubstrate. The process can be adapted such that the source materialcoats the substrate to form a continuous thin film. Such a film can beleft intact or removed from the substrate to form particles such assubstantially flat pigment particles

[0041] The apparatus and process of the invention are particularlysuited for producing flexible films having an optical interferencecoating with a very high surface thickness uniformity. In the field ofsolar control window film, the invention solves the problem of makingmultilayer coatings with acceptable uniformity, optical performance, andcost. The present invention also allows useful metal layers anddielectric layers of an optical coating to be deposited at highuniformity on a wide plastic web for the purpose of forming micron sizedpigment particles, by removing the coating from the web.

[0042] Other objects and advantages of the present invention will becomeapparent from the following descriptions, taken in connection with theaccompanying drawings, wherein, by way of illustration and example,various embodiments of the present invention are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to illustrate the manner in which the above recited andother advantages and objects of the invention are obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

[0044] FIGS. 1A-1C illustrate a prior art source in cross-section (FIG.1A) and perspective views (FIGS. 1B-1C);

[0045]FIGS. 2A and 2B illustrate a prior art source in cross-section andexploded views, respectively;

[0046] FIGS. 3A-3C are schematic depictions of a deposition apparatusaccording to one embodiment of the invention;

[0047]FIGS. 4A and 4B are graphs illustrating a predictive model forcross web coating thickness uniformity as a function of L/D ratio;

[0048]FIG. 5 illustrates the geometry used by the predictive model;

[0049]FIGS. 6A and 6B illustrate the model results graphically andfurther illustrate the model geometry;

[0050]FIG. 7 is a cross-sectional schematic view of a depositionapparatus according to another embodiment of the invention;

[0051]FIG. 8 is a cross-sectional schematic view of a depositionapparatus according to an additional embodiment of the invention;

[0052] FIGS. 9A-9D are schematic cross-sectional views of variousalternative embodiments of the invention, wherein the vapor flux isdirected either downward or horizontally;

[0053]FIG. 10 is a drawing from a photograph of a multi-layer coatingproduced by a prior art source illustrating non-coated areas, whichresult from approximately 1 cm diameter particulate shadowing thesubstrate;

[0054]FIGS. 11A and 11B are schematic views of additional embodiments ofthe invention, wherein a plurality of sources is utilized in series in avacuum coating machine for web coating (11A) and coating discrete flatsubstrates (11B);

[0055]FIG. 12 is a schematic plan view of an another embodiment of theinvention, wherein a plurality of sources communicates with a commonchimney in a vacuum coating machine;

[0056]FIGS. 13A and 13B show schematic views of a further embodiment ofthe invention, wherein several sources connect to a common chimney, thechimney directing a uniform vapor flux onto a vertical substrate;

[0057]FIGS. 14A and 14B show schematic views of another embodiment ofthe invention, wherein the chimney has a rectangular slot opening fordirecting a uniform vapor flux onto a vertical substrate;

[0058]FIGS. 15A and 15B are graphs which compare the observed cross webuniformity of ZnS as deposited with predictive model results; and

[0059]FIG. 16 is a graph which illustrates the observed down web coatingthickness uniformity of ZnS.

DETAILED DESCRIPTION OF THE INVENTION

[0060] The present invention is directed to an apparatus and process forthermal evaporation and deposition of materials uniformly on widesubstrates. The apparatus and process are especially applicable to theevaporation of materials that sublime, i.e., evaporate from asolid-state. During the sublimation of source materials, solid particlesand particulate tend to be ejected, a phenomenon commonly known asspatter. This phenomenon causes defects in the coatings, and is usuallyavoided by maintaining the power below a critical threshold, hencelimiting the deposition rate. The present invention provides essentiallydefect free coatings at very high deposition rates, wherein thedetrimental effects of spattered particles and particulate areeliminated by the inventive source design.

[0061] As described in further detail below, the various embodiments ofthe invention provide a variety of evaporation source configurations todirect vapor flux upward, sideways and/or downward onto a substrate. Theapparatus and process are readily scalable in the width of the coatingsto match the substrate and/or deposit a requisite coating thicknesswithout refilling the source between vacuum cycles.

[0062] A variety of heater configurations may be used with theevaporation source. The heater power supply and/or substrate drive areregulated by a control circuit responsive to a coating control monitorthat measures a property of the coating, which is indicative of the filmthickness. The novel features of the invention, which lead to highcoating uniformity, also result in a rapid response to the heater power.This permits the use of source power as well as substrate transportspeed (web speed) for temporal control of deposition rate, improving thedown web uniformity, without a deterioration in cross web uniformity.

[0063] The evaporation source is constructed and used in a manner thatfacilitates rapid startup and cool down, thus improving cycle time. Thisis accomplished by the source having a fast temporal response to changesin input power, permitting continuous control of the deposition rate andproviding the economic advantages of a short time for heating up (to thedeposition temperature during start-up) and cooling down (for re-loadingsubstrate and/or source material).

[0064] Referring to the drawings, wherein like structures are providedwith like reference designations, FIGS. 3A-3C are schematic views of adeposition apparatus according to one embodiment of the invention. FIG.3B is a plan view of the deposition apparatus, in which section lineA-A′ indicates the cross-sectional view depicted in FIG. 3A. Thedeposition apparatus includes a source 60 having a crucible 62 forcontaining a charge of source material 64, and a chimney 66 with a vaporinlet end 68 mounted on crucible 62. The chimney 66 has a rectangularvapor outlet slot 70 for directing source vapor from crucible 62 to asubstrate 72. A flow-restricting baffle 74 is provided between chimney66 and crucible 62 for blocking particulate ejected from source material64. A floating baffle 76 having a plurality of holes 78 is providedwithin crucible 62. The floating baffle 76 is adapted to maintain itsposition on the upper surface of source material 64, as source material64 sublimes. The holes 78 in floating baffle 76 are arranged inco-operative association with openings in flow restricting baffle 74 inorder to block particulate ejected from source material 64. The crucible62 has an upper surface or lid 80, which can be removed to insert orreplace floating baffle 76 after refilling source material 64.

[0065] A heating element 82, such as resistive heating rods, surroundscrucible 62 and is adapted to uniformly heat source material 64 to anevaporation or sublimation temperature. The crucible 62 is containedwithin a source box 84 having an inner surface of a refractory metalheat shield 86 surrounding heating element 82. The source box 84 mayalso include an insulating material 88 enclosing heat shield 86. Thesource box 84 may be enclosed within a containment and cooling vessel90. The vessel 90 has water cooling lines 92 on its outer surface, thussubstantially preventing excess heating of substrate 72 and other partsof the vacuum chamber (not shown).

[0066] The coating process occurs in a vacuum chamber (not shown)adapted for substrate entry, transport and removal. The vacuum chambermay operate in a batch mode, wherein the entire substrate 72 iscontained within the vacuum chamber during the entire compound coatingand vacuum venting cycle. Alternatively substrate 72 may be introducedinto the vacuum chamber continuously during the vacuum cycle andsequentially removed after coating. Continuous coating chambersintroduce and remove substrate from either isolated air lock chambers ordifferentially pumped zones that have constrictions which conform to aplanar substrate, such as a continuous web of plastic film or metalsheet.

[0067] During a coating cycle, source 60 operates in the followingmanner. The source material 64 is heated to the evaporation orsublimation temperature within a first region ({circle over (1)} in FIG.3A) in the interior of crucible 62. This first region {circle over (1)}is defined by one or more baffles that restrict the flow of vapor into asecond region ({circle over (2)} in the FIG. 3A), whereby the restrictedflow results in a significantly higher pressure in first region {circleover (1)} than in second region {circle over (2)}. The first and secondregions {circle over (1)} and {circle over (2)} are separated by aconduit, such as chimney 66, which confines and directs a plume of vaporonto substrate 72. The vapor plume that reaches substrate 72 isessentially free of particulate ejected by source material 64. The vaporflux within this plume is spatially and temporally uniform, with respectto a plane defined by the conduit width, due to the lower gasconductance through the conduit than within first region {circle over(1)}.

[0068] The floating baffle 76, which is disposed on top of the solidsource material 64, and flow restricting baffle 74 between first andsecond regions {circle over (1)} and {circle over (2)} cooperate toreduce the gas conductance from region {circle over (1)} to region{circle over (2)} and also to intercept a large quantity of ejectedparticulate when source 60 is operated at the maximum temperature. Thegas conductance difference is maintained as crucible 62 is emptied ofsource material 64, wherein the crucible volume filled with sourcematerial decreases from an initial value of about 98% to about 10% orless, over the course of a coating run.

[0069] The vapor outlet 70 at region {circle over (2)} may be placedclose to substrate 72 to achieve efficient material utilization; i.e.,substrate 72 is coated rather than the vacuum chamber walls, without anysacrifice in transverse uniformity. The substrate is not unduly heatedby the source, since the narrow slit width at outlet 70 provides minimaldirect infrared (IR) irradiation from the hot evaporation chamber.

[0070] The foregoing process can be carried out utilizing numerousvariations in structural details and process conditions, examples ofwhich are provided herein. In alternative embodiments, heating elements82 may be within crucible 62, and may comprise one or more (IR) sources,such as IR lamps or SiC glow bars. Alternatively, the discrete heatingelement may be eliminated when the crucible functions as a resistiveheating element, i.e., when the crucible is conductive and supplied witha current adapted to heat the crucible and source material, at theappropriate voltage for the crucible's electrical conductance.Alternatively, inductive heating may be used when the source materialand/or crucible are conductive. Preferably, the heaters are arranged toprovide uniform heating over the width of the source to obtain the bestcoating uniformity.

[0071] The inventive source design is surprisingly tolerant ofnon-uniform heating because flow restricting baffle 74 substantiallyequalizes any spatial variance in the vapor pressure within crucible 62,which would otherwise result in a correspondingly non-uniform vapor fluxof source material 64 caused by local temperature variations withinsource material 64. Therefore, it is not essential to provide a largethermal mass, of either source material 64 or crucible 62, to obtain aconsistent and uniform deposition rate. In fact, a low thermal masssource and heating element are advantageous, in that the source can beheated and cooled very rapidly, decreasing the non-productive cycle timewhen either reloading source material or replacing coated substrate withbare substrate.

[0072] In a currently preferred arrangement, molybdenum (Mo) heatingrods are wrapped around the crucible. The preferred Mo heating rods havea 5 mm diameter and are typically provided with about 200 amps per 12inches of linear width at 12 volts, which allows heating of zinc sulfideto approximately 1000° C., the useful sublimation temperature.

[0073] Depending on the material evaporated, chimney 66 may be heated,as shown in FIG. 3A, to reduce the material's sticking coefficient,thereby preventing the deposit of source material 64 along the interiorwalls of chimney 66 which would degrade the coating uniformity.

[0074] The heat shield 86 is preferably formed from an Mo sheet 0.02inch thick, but can generally range from about 0.01 to about 0.05 inchthick, a sufficient thickness for dimensional stability, but avoiding amaterial thickness that would retain and radiate excess heat. The heatshield 86 is further insulated on the outer surface thereof byinsulating material 88, such as a fibrous alumina board or a carbon feltcomposite material. The insulating material 88 is separated by vacuumaway from and surrounded by containment and cooling vessel 90, such as acopper box. The temperature of this copper box is regulated by watercooling lines 92, which are attached coils continuously flushed withcooling water.

[0075] It has been surprisingly found that the present process andproduct can be optimized within the following range of structuraldimensions of the source and their relationship to the substrate. FIG.3C provides details of a portion of section A-A′ in FIG. 3B, definingstructural dimensions and parameters which are optimized in the morepreferred embodiments of the invention. The distance from the top ofvapor outlet slot 70 of chimney 66 to substrate 72 is designated as D.The height of chimney 66, i.e., the distance from vapor inlet end 68 tothe top of vapor outlet slot 70 is designated as H. The width of chimney66 is designated as W1 and the width of crucible 62 is designated as W2.FIG. 3B defines L, the length of vapor outlet slot 70 in the cross webdirection, i.e., transverse to the direction of substrate transport(down web direction), as illustrated by arrows in FIG. 3B.

[0076] It has been surprisingly found that coating thickness uniformityin the direction transverse to substrate transport (the cross webdirection) is optimized by the ratio of L/D. The ratio L/D is preferablygreater than about 8, more preferably greater than about 16, and mostpreferably greater than about 32. The ratio of W2/W1 also contributes tooptimum coating thickness uniformity. The down- and cross-web uniformityis improved when W2/W1 is preferably greater than about 3, morepreferably greater than about 4, and most preferably greater than about8. The ratio of H/W1 also contributes to cross web uniformity, as wellas to down web uniformity. H/W1 is preferably greater than about 5, morepreferably greater than about 8, and most preferably greater than about20.

[0077] Not wishing to be bound by theory, we believe that a higher ratioof H/W1 contributes to the cross web uniformity. When H/W1 is large,there is a greater probability that molecules of source material vaporwill collide with the chimney walls or with other molecules, equalizingregions of higher and lower pressure in the slot, and resulting in adirected vapor plume exiting the chimney. The source appears to berather tolerant of non-uniform source material heating, which wouldnormally result in a non-uniform vapor flux exiting the chimney.

[0078] The optimum ratio of L/D is illustrated in FIGS. 4A and 4B,differing substantially from the teaching of the prior art that L/D<1 isdesirable and better uniformity is obtained as L/D decreases. Theassumption behind this model is that a slot source can be modeled as anarray of many point sources. Each of these point sources generates anequal amount of vapor. The vapor ejected from these sources isdistributed according to a cosine law, where the probability of a vapormolecule escaping at a particular trajectory is proportional to cos².The vapor cloud from a specific source decays according to an inversesquare law with distance from the source. The vapor impacting with thesubstrate is scaled by the cosine of the deposition angle to compensatefor the flux. In this model, no scattering is taken into account. Adiagram of this geometry is shown in FIG. 5.

[0079] The model has a single parameter, which is the ratio of sourcelength (L) to source distance (D) from web (L/D). All results given aredimensionless. The deposition is expressed as a percentage of maximum oraverage. The cross web position is specified as a percentage of thelength L.

[0080] The total deposition (W) at each substrate location is determinedby the following summation, where n is large and represents the numberof point sources used in the model.$W = {\sum\limits_{i = 1}^{n}\frac{\cos^{3}\theta}{r^{2}}}$

[0081]FIG. 4A is a contour plot, which graphically shows the level ofcross web uniformity that can be expected as a function of the L/Dratio. The different contour regions show the amount of materialdeposited across the web (in the crossweb direction) as a percentage ofthe average deposition. Thus, the regions marked “95-100” and “100-105”are essentially uniform at the average deposition, whereas regions withhigher or lower percentages represent excessive or insufficientdeposition in local regions, i.e., nonuniformity. The plot illustratesthe model predictions, in which the best cross web uniformity will occurat either large or small values of L/D.

[0082] When L/D is large, the source is very close to the substrate,resulting in a uniform vapor flux and a high utilization of sourcematerial. For example, looking at the horizontal line representingL/D=64, the deposition is uniform at 100-105% of the average from 0.05to 0.95 of the crossweb direction; i.e., over the central 90% of theweb. At the edges of the web (<0.05 and >0.95 of the crosswebdirection), the deposition is only slightly less, 95-100%. Similarly, atsmall values of L/D the deposition is uniform. For example, looking atthe horizontal line for L/D=0.0625, the deposition is uniform at100-105% from 0.2 to 0.8 of the crossweb direction (the central 60%),and slightly less at the edges. At small values of L/D, the source isfar away from the substrate and acting as a point source.

[0083] It should be noted that at moderate values of L/D from about 0.5to about 8, the cross web uniformity would be very poor, requiringmasking for further improvement. Thus, for example, looking at thehorizontal line for L/D=1, the deposition is 80-85% from 0 to 0.05,85-90% from 0.05 to about 0.1, 90-95% from 0.1 to 0.15, 95-100% from0.15 to 0.2, 100-105% from 0.2 to about 0.27, 105-110% from 0.27 to0.37, 110-115% from 0.37 to 0.62, and then decreases symmetrically backto 80-85% at the other edge.

[0084]FIG. 4B shows an alternative representation of this data. Thisplot shows the range of uniformity over the center 90% of the substratefor a large range of L/D ratios. With this plot, a given natural sourceuniformity can be associated with a required L/D ratio to achieve thatuniformity. For example, to achieve less than a 1% non-uniformity with aslot source configuration would require an L/D ratio of greater thanabout 20-25. The experimental agreement with this model will bedescribed further in the Example herein, but can be found in FIG. 11.

[0085] The utilization of source material on a round drum can bemaximized when the ratio D/D′ is maximized, where D is the distance fromthe chimney outlet to the substrate, and D′ is the drum diameter. Theresults of modeling are depicted in FIG. 6A, which plots the %utilization of the source material that reaches the substrate as afunction of D/D′. The model is calculated by solving the integralequation representing the source distribution flux (U) reaching thesubstrate, integrated over the substrate area:$U = \frac{\int_{0}^{\theta_{\max}}{\cos^{2}\theta {\theta}}}{\int_{0}^{\pi/2_{\max}}{\cos^{2}\theta {\theta}}}$

[0086] Referring to FIG. 6B, θ is the angle of incidence having amaximum value θ_(max) representing the maximum angle at which materialleaving the source reaches a portion of the drum. The angle θ_(max) isdefined by a line from the chimney outlet to a point tangent to the drumsurface, and is given by:$\theta_{\max} = {\tan^{- 1}\frac{D^{\prime}\sin \quad \phi}{{2D} + D^{\prime} - {D^{\prime}\cos \quad \phi}}}$

[0087] where D and D′ are as defined above, and φ is the angle oftangency with respect to the drum, given by:$\phi = {\cos^{- 1}\frac{D^{\prime}}{{2D} + D^{\prime}}}$

[0088] Integrating the equation, the fraction of source material that isutilized or deposited on the substrate is given by:$U = {{\left( \frac{2}{\pi} \right)\theta_{\max}} + {\left( \frac{1}{\pi} \right)\sin \quad \left( {2\quad \theta_{\max}} \right)}}$

[0089] As an ideal model, which does not account for source vapor lossfrom leakage of the crucible or source box, back scattering in thedeposition zone, or a sticking coefficient less than unity, the modelrepresents a maximum possible utilization, not an absolute result.

[0090] With the recognition of the significance of these variables, thepresent invention provides preferred configurations of the crucible andchimney structures to provide for their spatial and temporal stability.Specifically, the chimney should not distort in shape nor vary indistance from the drum or substrate during a coating run, which wouldmodify W1 and D. Shape distortion has been avoided by stiffening theupper edge of the chimney with flared edges 94, as shown in FIG. 3A.Alternatives are ribs or other structures that are known to preventdistortion from thermal expansion of a metal sheet, or using thermalexpansion joints between source components.

[0091] In order to prevent reduction in D or W1 by the condensation ofsolid source material, either within or on the chimney, the chimney isoptionally heated. The heating source can be either supplemental heatingelements or a common heating element. It will be appreciated by thoseskilled in the art that if the chimney is heated by conductive heattransfer from the crucible, then the chimney temperature need onlyincrease to a temperature at which the sticking coefficient of thesource material vapor is sufficiently low. This requirement is thereforesource material specific, and can readily be evaluated by varying thepower to a supplemental heater such that a coating deposit does not formon the chimney 66 surfaces. This prevents a deposited coating fromforming and acting as a physical mask to the coating of substrate 72.

[0092] Typically the distance D between the top of the chimney 66 andthe substrate 72 is about {fraction (7/16)} in.

[0093] Returning to FIG. 3A, flow restricting baffle 74 preferably hasholes of about 2 mm in diameter at a 1 cm×0.5 cm center-to-centerspacing, resulting in an open area of about 7%. The floating baffle 76has holes 78 that are smaller than the source material 64 particles,with the holes typically having a diameter of about 2 mm spaced at abouta 5 mm center-to-center spacing, for an open area of about 12%.

[0094] The configuration of holes 78 in floating baffle 76 has a spacedrelationship with flow restricting baffle 74, to substantially avoidline of site transmission of spatter particles from source material 64into chimney 66. The floating baffle 76 does not have any holes in theregion immediately perpendicular to the holes in flow restricting baffle74. Further screening of spatter particles is achieved by adjusting theflow restricting baffle 74 hole size and orientation. Mesh screen may beadapted to form flow restricting baffle 74. The flow restricting bafflecharacteristics can be readily optimized for the spatter characteristicsof different source materials by combining multiple screens or forms ofpunched metal sheet.

[0095]FIG. 7 is a cross-sectional schematic view of a depositionapparatus according to another embodiment of the invention, which has analternative crucible, chimney and substrate configuration. Thisconfiguration substantially stabilizes the width (W1) of a chimney 166and spacing (D) from a substrate 72. An exit opening of a crucible lid180 is formed by an integral conduit 181. The chimney 166 is mounted ina coating chamber (not shown) by a bracket 200 and loosely fits overconduit 181, which forms the exit of crucible lid 180. In thisembodiment, crucible lid 181 contains a flow restricting baffle 174.Heat conduction from a crucible 162 to chimney 166 is minimized,reducing the chimney temperature and preventing thermal distortion ofthe chimney shape or opening. Uniformity is thus improved by the rigidpositioning of the chimney outlet.

[0096]FIG. 8 illustrates an alternative embodiment of the depositionapparatus of the invention having elements similar to the embodiment ofFIG. 7, including a chimney 166 mounted in a coating chamber by abracket 200. The embodiment of FIG. 8, however, has an alternativearrangement of a flow restricting baffle 274 with respect to chimney166. As shown in FIG. 8, the openings of flow restricting baffle 274 maybe suitably provided on a surface of an integral conduit 281 of a lid280 that extends into a crucible 262. This substantially eliminates a“line of site” path between any openings in chimney 166 and the openings178 in a floating baffle 176, substantially preventing spatteredparticulate from entering chimney 166.

[0097] FIGS. 9A-9D are schematic cross-sectional views of variousalternative embodiments of the invention for directing the vapor stream(horizontally or vertically) and independently controlling the chimneytemperature. In FIG. 9A, a chimney 266 is not attached to a crucible362, but has its vapor inlet end 168 connected to the cavity formedbetween crucible 362 and a metal heat shield 186. The chimney 266penetrates metal heat shield 186 and insulating material 188, which forma source box 184, and the containment and cooling vessel (not shown inthis figure) at the bottom of the source. This configuration results ina downward flow of source material vapor onto the top of a horizontallydisposed substrate 172. A flow restricting baffle 374 is still required,and is exposed on the vapor inlet end 168 of chimney 266. A cruciblelid, not shown, is optional, depending on the configuration of holes 178in a floating baffle 176, which can be disposed in a cooperativerelationship thereto, preventing particulate ejected from the top ofcrucible 362 from entering chimney 266 at a velocity sufficient to reachsubstrate 172. Another optional variation is also illustrated, in whichadditional insulating material 189 surrounds chimney 266 to maintain thechimney near the source material temperature, preventing a coatingdeposit from forming within the chimney.

[0098] In FIG. 9B, a chimney 366 is disposed horizontally, penetratingmetal heat shield 186, insulating material 188 and the containment andcooling vessel (not shown), at their respective side walls. Thisconfiguration results in a horizontal flow of source material vapor ontothe surface of a vertically disposed substrate 272 being translated in ahorizontal direction.

[0099] It may be necessary to increase the chimney temperature toprevent deposition either inside the chimney or on the outlet surface.As will be recognized by one of skill in the art, the preferred chimneytemperature is specific to both the source material and the depositionconditions. The chimney temperature can be increased by exposing agreater portion of the chimney's length to the heater elements withinthe source box, by adding heater elements, by reducing the chimneylength, by re-positioning the crucible, and the like.

[0100]FIGS. 9C and 9D illustrate further alternative embodiments havingparticular utility when it is necessary to reduce the temperaturedifference between the chimney and the source material in the crucible.In FIG. 9C, a chimney 466 is thermally coupled to a crucible 462.Thermal coupling will reduce the chimney temperature, when the chimneyis hotter than the source material, as crucible 462 is cooled byevaporation of the source material. FIG. 9D illustrates an embodimentfor use when it is necessary to prevent the source vapors fromover-heating, by direct exposure to the heating elements. A chimney 566is connected directly to a crucible 562 at a point above the sourcematerial 64, thus fully containing the source vapor and directing itdownward. The embodiment of FIG. 9D can be modified for horizontaldeposition.

[0101] It should be noted for the above described embodiments that whenthe coating source material is molten within the crucible, the floatingbaffle is generally not required. When a liquid evaporates from thesource it is permissible for vapor to condense as liquid on the interiorwalls of the chimney, in which case it will flow back down into thecrucible.

[0102] Normally the ejected particulate, or spatter, is microscopic insize and will increase the roughness of the film surface in conventionalprocesses, which can, under extreme conditions, result in a hazyappearance of the coated substrate. Occasionally, the spatter particlesare sub-millimeter in size, thus clearly visible to the naked eye. Thisis generally acceptable for applications wherein the final film productis laminated with adhesive either inside glass panels or onto thesurface of another substrate. However, for computer display applicationsthat have a resolution of less than about 0.25 mm, even spatterparticles or defects less than a millimeter in diameter would not beacceptable. Under the highest deposition array conditions, the particlesejected from zinc sulfide are significantly larger, about 5-15 mm indiameter, and roughly shaped. When these larger particles hit thesubstrate, they shadow the substrate from the instantaneous vapor flux,which results in large visible streaks of uncoated substrate. FIG. 10illustrates by way of a drawing from a photograph a multilayer coatingproduced by the prior art source shown in FIG. 2 and described above.The streaks are outlined and numbered 1-8. The particles and theresulting defects range in size from 5 to 20 mm in diameter. Thesedefects are clearly unacceptable for almost any end use application.

[0103]FIGS. 11A and 11B are schematic views of coating systems accordingto the invention that utilize a plurality of sources in series in avacuum coating machine. The sources utilized in the coating systems canbe selected from any of the embodiments previously described. In FIG.11A, a plurality of sources 160 a, 160 b and 160 c are utilized inseries in a vacuum coating machine 300 for coating a continuous web 302,arranged around a drum 304, maximizing the number of deposition zones.

[0104] In FIG. 11B, a series of source boxes 260 a and 260 b arearranged horizontally in a coating machine 400 having load lock entryand exit chambers 402 and 404 for coating flat discrete parts, such asglass sheets 406. The entry and exit chambers 402 and 404 are isolatedfrom a processing chamber 408 by vacuum locks 410. Each source 260 a and260 b is provided with a separate heater and heater control circuit (notshown) and shutters 412 a and 412 b. The shutter prevents depositiononto an empty portion of the substrate carrier. The glass sheets 406 aretransported by a series of conveyor belts 414.

[0105]FIG. 12 is a plan view of a further embodiment of the inventionwherein a plurality of sources 360 a, 360 b and 360 c are utilized inparallel in a vacuum coating machine (not shown). Each source 360 a, 360b and 360 c is provided with a separate heater and heater controlcircuit (not shown). The three sources communicate with a common chimney666.

[0106]FIG. 13A is a cross sectional view of an additional embodiment ofthe invention, wherein a plurality of sources 460 a-460 e communicatewith a common chimney 766 to deposit a vapor stream onto a verticalsubstrate 372. A vapor outlet slot, not shown, is disposed vertically toallow for deposition on a substrate 372 that is moving in the verticaldirection. FIG. 13B is a cross-sectional view along section line A-A′ inFIG. 13A, showing a crucible 662 along with a floating baffle 176 foruse therein, and a flow restricting baffle 474 between crucible 662 andchimney 766.

[0107]FIG. 14A is a cross-sectional view of a further embodiment of theinvention wherein a coating is deposited from a single source 560,having a single crucible 762 with a floating baffle 176. A chimney 866extends vertically, and has a vertically disposed flow restrictingbaffle 474. The chimney has a vertical opening (not shown) to deposit acoating material onto a substrate 472, which is held vertically andtransported vertically, in this case on a rotating drum 406 utilized asthe substrate carrier. A series of heater elements 182 are provided tomaintain chimney 866 at a temperature sufficient to prevent the coatingmaterial from depositing within the chimney. FIG. 14B is across-sectional view along section line A-A′ in FIG. 14A, showingchimney 866 and crucible 762 with flow restricting baffle 474therebetween.

[0108] The aforementioned combinations of elements in the variousembodiments of the invention, result in sources providing a highdeposition rate, high thickness uniformity, and avoidance of spatter.These advantages are mutually achieved by a combination of source designfeatures in a cooperative relationship with the substrate. Notably, thepresent invention demonstrates advantages over state-of-the-art coatingtechnologies in deposition rate, coating uniformity, materialutilization, energy consumption, process time, and coating quality. Forexample, prior to this invention, the coating industry lacked thecapability of producing high surface quality optical coatings, such ascoatings comprised of zinc sulfide (ZnS), having a uniform color overthe width of standard plastic web substrate. The invention provides thecapability for achieving coating thickness uniformity of better thanabout 5% over a 40-inch or greater substrate width, both perpendicularand parallel to the substrate transport direction.

[0109] The present invention solves the problem of simultaneousimprovement in coating quality and economy, especially in coatingscontaining multiple layers. Specifically, it enables the deposition ofhigh quality coatings at a high rate. Coatings of zinc sulfide are ofparticular note as an example of a material with high refractive indexthat can be deposited at high rates with high coating quality by usingthe present invention. This is advantageous since zinc sulfide is auseful high index material for constructing a wide variety of opticalthin-film coating designs. Other examples of coating materials that canbe deposited by the apparatus of the invention are chromium (Cr),silicon dioxide, magnesium fluoride (MgF₂), and cryolite; this list ofmaterials is by no means exhaustive.

[0110] The apparatus of the invention is particularly useful indepositing zinc sulfide, magnesium fluoride, and various oxides ofsilicon (SiO_(x)), such as silicon dioxide (x≈2), silicon monoxide (x≈1)and suboxides (x<2), onto a substrate comprising plastic film withoutexcessive heating and distorting of the film. The apparatus may also beused to deposit materials that evaporate from molten or liquid state.

[0111] The present invention is particularly suited for making aflexible film having an optical interference coating, with the coatingcomprising at least one layer of material such as those described above.The at least one layer of the coating has a thickness that varies byless than about 3%, and preferably by less than about 1.5%, across adistance of at least about 12 inches, and preferably across a distanceof at least about 40 inches. In a preferred embodiment, a flexible filmmade according to the invention has a thickness that varies by less thanabout 1% across a distance of at least about 60 inches. When theflexible film is formed such that the at least one layer of the coatingis deposited from a solid source material by sublimation, the coating isessentially free of defects of average diameter greater than about 10mm, preferably greater than about 5 mm, and more preferably greater thanabout 1 mm, caused by ejection of particulates from the source material.

[0112] The present invention addresses the growing market need forenergy (solar) control film in automotive and architectural markets.Energy control films can be laminated between window glass or placedwithin an evacuated space between window panel frames. A highly uniformcoating is required to achieve a uniform and aesthetically pleasingreflected or transmitted color for many of these applications. Solarcontrol films used for automotive glazing should exhibit uniformity ofboth the reflected and transmitted color across a polyester web 12 ormore inches wide. This will generally require that each high indexmaterial layer have a thickness that varies less than about ±3%,preferably less than about ±1.5%, and more preferably less than about±1%. Solar control films for architectural glass usually require coloruniformity across a polyester web greater than about 20 inches wide,preferably greater than about 40 inches wide, and more preferablygreater than about 60 inches wide. The present invention provides thecapability for meeting the above requirements for solar control windowfilm, solving the problem of making multi-layer coatings with acceptableuniformity, optical performance, and cost. The coatings made by theinvention have optical constants desirable for application in solarcontrol films, wherein a near bulk property refractive index is obtainedwithout significant optical absorption.

[0113] The present invention is particularly useful in depositing zincsulfide as an optical coating material with a refractive index greaterthan about 2.2 and an absorption coefficient less than about 0.01,preferably less than about 0.001, and more preferably less than about0.0003, at a visible wavelength of 550 nm, in a multi-layer solarcontrol coating on a polyester film substrate. Examples of suitableenergy control multi-layered coatings composed of zinc sulfide which canbe deposited on plastic film or web substrates utilizing the apparatusand process of the invention, can be found in U.S. Pat. No. 4,536,998 toMatteucci et al., and U.S. Pat. No. 5,677,065 to Chaussade et al., thedisclosures of which are herein incorporated by reference.

[0114] Another suitable optical coating which can be deposited utilizingthe apparatus and process of the invention is described in U.S. Pat. No.4,229,066 to Rancourt et al., the disclosure of which is hereinincorporated by reference. The optical coating is a visiblytransmitting, infrared reflecting filter that includes zinc sulfide as ahigh index material, which is advantageous to deposit on silicon typesolar cells that frequently come in a fused silica or glass cover sheet.

[0115] Other suitable optical coatings which can be deposited utilizingthe apparatus and process of the invention include zinc sulfide coatingson plastic film used to form anti-reflection films which can belaminated to the front face of various information display panels, suchas cathode ray tubes and liquid crystal display panels. A furtherdescription of such optical coatings is found in EP 539,099 A2, thedisclosure of which is herein incorporated by reference.

[0116] The apparatus and process of the invention can also be utilizedin the formation of various pigment materials, such as those describedin the following patents. For example, U.S. Pat. No. 3,123,489 toBolomey et al., the disclosure of which is incorporated by reference,describes how nacreous pigment can be made by evaporation of ZnS onto aflexible substrate and removal therefrom, forming pigment flakes. U.S.Pat. No. 5,648,165 to Phillips et al., the disclosure of which isincorporated by reference, describes how optically variable flakes andcoatings can be produced by depositing a multi-layer coating on aplastic film and then removing the coating from the film. Themulti-layer materials are, for example, zinc sulfide and magnesiumfluoride or silicon dioxide. This same patent describes how opticallyvariable pigments can be formed using a five-layer symmetrical design ofthe type:

[0117] metal/dielectric/metal/dielectric/metal.

[0118] When using the various embodiments of the invention to coatdielectric substrates, such as polyester film or glass, it is necessaryto remove static charge buildup on the substrate to deposit high-qualitycoatings such as a zinc sulfide coating. Without treating dielectricsubstrates in this manner, the coatings are of lower quality and have amottled appearance, suggesting it is structurally or chemicallyinhomogeneous. These generally undesirable characteristics are moreprominent as the coating thickness is decreased, corresponding to lowerdeposition rate. Not wishing to be bound by theory, we believe the filmquality is related to the nucleation rate, in that nucleation issuppressed by the residual static charge. A static charge on thesubstrate would repel one of the ionized species (e.g., Zn when thestatic charge is positive). Nucleation and growth require that eitherboth ions, or ZnS molecules, are present at the gas-surface interface.The degradation in film quality at the lower deposition rates suggeststhat either the ionized species are rate limiting or the excess ionsbecome incorporated in the film growing from ZnS molecules, disruptingthe film's structure. The static charge is easily removed with a glowdischarge on plastic or glass substrate. Metallic or metal-coatedsubstrates, when they are sufficiently conductive, do not accumulate astatic charge, obviating the need for a glow discharge treatment.

[0119] The following example is given to illustrate the presentinvention, and is not intended to limit the scope of the invention.

EXAMPLE 1

[0120] In this example, a floating baffle and flow restricting bafflewere utilized as shown in the embodiment of FIG. 3C. The crucible wasformed from a rectangular box having dimensions 2×2×9.5 in. Thedimensions of the other source components were: L 9.5 in. H 3.5 in. W10.625 in. Crucible width, W2 2 in. Distance from chimney 0.437 in. tosubstrate, D Drum diameter, D′ 11.8 in. (30 cm) L/D ratio 21.7 H/W1ratio 5.6 W2/W1 ratio 3.2 D/D′ 0.037

[0121] The flow restricting baffle contained five rows of 3-mm holes toprevent ZnS particulate from being ejected from the source material. Thesubstrate was polyester film having a thickness of 0.002 in. Thepolyester had been aluminized to facilitate coating thicknessmeasurements. There was no shutter between the source and the substrate,nor was there any masking.

[0122] Base pressure was 5×10⁻⁵ Torr. The stability and sourceuniformity were evaluated at two conditions, denoted “A” and “B”. Incondition A, 1.4 kW of power was applied to the source, and the webtransport speed was 0.5 m/min. In condition B, the power was increasedby a factor of 1.66 (a 66% increase in power) to 2.33 kW. The web speedwas increased to 3 m/min, to achieve a coating thickness that could bemeasured with sufficient accuracy.

[0123] Cross web coating uniformity representative of condition is shownin the graph of FIG. 15A. The coating uniformity was within ±1%,excluding about 1.5 inches at both edges of the polyester film, inexcellent agreement with the model for thickness uniformity, plotted asthe solid line in the graph of FIG. 15B. The results of the model wereabove described and shown in the graphs of FIGS. 4A and 4B.

[0124] Down web uniformity for conditions A and B is illustrated in thegraph of FIG. 16. The thickness measurement of the ZnS was taken at thecenter of the web and is plotted against the down web distance, inmeters. The average deposition rate in condition A was 0.55micron/m/min, and in condition B was 3.6 micron/m/min. The highfrequency variations are due to instability in the web drive mechanismand are not a characteristic of the process. The longer-term variationsin rate, over hundreds of meters, for the duration of condition A and B,are easily eliminated with a conventional control system. Thediscontinuous rate increase from region A to region B indicates theresponsiveness of the source to the heater power, with the depositionrate under condition B (2.33 kW) increasing to about seven times that ofcondition A (1.66 kW). The subsequent decrease in coating rate, over webdistance, in region B is a consequence of the rapid depletion of thesource material in the crucible, which occurs over a shorter webdistance at the higher deposition. The rate of decrease under conditionB appears to be significant with respect to region A only because theweb speed was increased by a factor of 6 in condition B. The initialsource material charge was about 1 kg of ZnS. At the end of theexperiment, about 25 g of ZnS remained in the crucible. The total weightof material deposited on the web during conditions A and B showed that55% of the ZnS was utilized and deposited on the substrate.

[0125] The present invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed and desired to be secured by United States Letters Patent is:
 1. A linear aperture deposition apparatus for coating a substrate, comprising: (a) a source box containing a charge of source material; (b) a heating element within the source box adapted to heat the source material to produce a vapor of the source material; (c) a chimney having at least one inlet in communication with the source box and a rectangular slot outlet for directing the vapor from the source box to the substrate; (d) a baffle disposed within the source box and configured to restrict the flow of vapor from the source box to the substrate; and (e) a containment and cooling vessel disposed around the source box and configured to prevent heating of the substrate.
 2. The linear aperture deposition apparatus of claim 1 , wherein the source material is contained within a crucible disposed within the source box.
 3. The linear aperture deposition apparatus of claim 1 , further comprising a floating baffle having a plurality of holes therethrough, the floating baffle adapted to maintain a position on an upper surface of the source material as the source material evaporates.
 4. The linear aperture deposition apparatus of claim 1 , wherein the chimney outlet protrudes from a side surface of the source box.
 5. The linear aperture deposition apparatus of claim 1 , wherein the chimney outlet protrudes from a bottom surface of the source box.
 6. The linear aperture deposition apparatus of claim 1 , wherein the baffle is disposed so as to substantially prevent particulate ejected from the source material from passing through the chimney outlet.
 7. The linear aperture deposition apparatus of claim 1 , wherein the chimney has a height H, the rectangular slot output has a width W1, and the ratio of H/W1 is greater than about
 5. 8. The linear aperture deposition apparatus of claim 7 , wherein the ratio of H/W1 is greater than about
 8. 9. The linear aperture deposition apparatus of claim 7 , wherein the ratio of H/W1 is greater than about
 20. 10. The linear aperture deposition apparatus of claim 2 , wherein the rectangular slot output has a width W1, the crucible has a width W2, and the ratio of W2/W1 is greater than about
 3. 11. The linear aperture deposition apparatus of claim 10 , wherein the ratio of W2/W1 is greater than about
 4. 12. The linear aperture deposition apparatus of claim 10 , wherein the ratio of W2/W1 is greater than about
 8. 13. The linear aperture deposition apparatus of claim 1 , wherein the rectangular slot output has a length L and is disposed at a distance D from the substrate, and the ratio of L/D is greater than about
 8. 14. The linear aperture deposition apparatus of claim 13 , wherein the ratio of L/D is greater than about
 16. 15. The linear aperture deposition apparatus of claim 13 , wherein the ratio of L/D is greater than about
 32. 16. A linear aperture deposition apparatus for coating a substrate, comprising: (a) a crucible containing a charge of source material and disposed within a source box, the crucible having a width W2; (b) a first baffle having a plurality of holes therethrough and adapted to maintain a position on an upper surface of the source material as the source material evaporates; (c) a heating element within the source box adapted to heat the source material to produce a vapor of the source material; (d) a chimney having at least one inlet in communication with the source box and a rectangular slot outlet for directing the vapor from the source box to the substrate, the chimney having a height H and the rectangular slot outlet having a width W1, a length L, and being disposed a distance D from the substrate; (e) a second baffle disposed within the source box and configured to restrict the flow of vapor from the source box to the substrate; and (f) a containment and cooling vessel disposed around the source box and configured to prevent heating of the substrate.
 17. The linear aperture deposition apparatus of claim 16 , wherein the ratio of H/W1 is greater than about
 5. 18. The linear aperture deposition apparatus of claim 16 , wherein the ratio of H/W1 is greater than about
 8. 19. The linear aperture deposition apparatus of claim 16 , wherein the ratio of H/W1 is greater than about
 20. 20. The linear aperture deposition apparatus of claim 16 , wherein the ratio of W2/W1 is greater than about
 3. 21. The linear aperture deposition apparatus of claim 16 , wherein the ratio of W2/W1 is greater than about
 4. 22. The linear aperture deposition apparatus of claim 16 , wherein the ratio of W2/W1 is greater than about
 8. 23. The linear aperture deposition apparatus of claim 16 , wherein the ratio of L/D is greater than about
 8. 24. The linear aperture deposition apparatus of claim 16 , wherein the ratio of L/D is greater than about
 16. 25. The linear aperture deposition apparatus of claim 16 , wherein the ratio of L/D is greater than about
 32. 26. A process for physical vapor deposition of a source material onto a substrate, the process comprising the steps of: (a) providing a source material within a source box; (b) heating the source material to form a vapor; (c) restricting the flow of vapor out of the source box to form a plume of vapor substantially free of solid particulate source material; and (d) transporting the substrate across the vapor plume to cause solid source material to coat the substrate.
 27. The process of claim 26 , wherein the source material coats the substrate to form a continuous thin film.
 28. The process of claim 27 , further comprising removing the film from the substrate and forming particles therefrom.
 29. A substantially flat pigment particle produced by the method of claim 26 .
 30. A flexible film having an optical interference coating, the coating comprising at least one layer of material selected from the group consisting of zinc sulfide, silicon oxides, magnesium fluoride, cryolite, and chromium, the at least one layer having a thickness that varies by less than about 3% across a distance of at least about 12 inches.
 31. The flexible film according to claim 30 , wherein the thickness varies by less than about 1.5% across a distance of at least about 12 inches.
 32. The flexible film according to claim 30 , wherein the thickness varies by less than about 3% across a distance of at least about 40 inches.
 33. The flexible film according to claim 30 , wherein the thickness varies by less than about 1% across a distance of at least about 60 inches.
 34. A flexible film having an optical interference coating, the coating comprising at least one layer deposited from a solid source material by sublimation, the coating being essentially free of defects of average diameter greater than about 10 mm caused by ejection of particulates from the source material.
 35. The flexible film according to claim 34 , wherein the source material is selected from the group consisting of zinc sulfide, silicon dioxide, silicon monoxide, silicon suboxides, magnesium fluoride, cryolite, and chromium.
 36. The flexible film according to claim 34 , wherein the coating is essentially free of defects of average diameter greater than about 5 mm caused by ejection of particulates from the source material.
 37. The flexible film according to claim 34 , wherein the coating is essentially free of defects of average diameter greater than about 1 mm caused by ejection of particulates from the source material. 